REDES ELÉCTRICAS

THERMAL ATOMIC LAYER DEPOSITION OF TUNGSTEN CARBIDE FILMS FROM WCl6 AND AlMe3

Reprinted (adapted) from Blakeney, K. J.; Winter, C. H. J. Vac. Sci. Technol. A 2018, 36, 01A104, with the permission of the American Vacuum Society

4.1 Introduction

Tungsten carbides (W2C, WC; hereafter WCx) are refractory materials with very high

hardnesses, melting points, and chemical resistance.45,94 Thin films of WCx have been

investigated as diffusion barriers and adhesion layers for Cu interconnects in integrated circuits.51,177,178 WCx thin films are typically grown by physical vapor deposition (PVD) using W

metal and methane177,178 or a WC target,179–181 or by chemical vapor deposition (CVD) using a variety of molecular tungsten precursors.49–53 Future applications in microelectronics devices will require precise film thickness control and perfect conformality in high aspect ratio features, in order to meet increasingly strict demands associated with smaller device dimensions.6,7

Atomic layer deposition (ALD) is a thin film deposition process that enables sub-nanometer control of film thickness and inherently gives perfect conformal coverage of shaped features because of the self-limited growth mechanium.6,7 There have been limited reports of WCx film

growth by ALD methods. ALD growth of WCx films was achieved at 250 °C using

W(NtBu)2(NMe2)2 with a hydrogen plasma,182 and use of W(CO)(EtC≡CEt)3 with a 10:1 H2:N2

plasma at 250 °C afforded WCx films.183 However, plasma-based ALD processes can lead to

substrate damage from the reactive plasma species and may afford poor conformal coverage in high aspect ratio features because the plasma constituents react with the feature walls before they reach the bottom of the feature.20 Brief mentions of the thermal ALD of WCx films appeared in

two recent review articles.17,24 _{This approach used WF}

have been several reports in the patent literature about the ALD growth of early transition metal carbide films using AlMe3 as the carbon source.184 Only recently has this method been studied

and disclosed in the scientific literature. An ALD process for tantalum carbide (TaCx) films was

briefly described in a 2007 review article.17 The TaCx films were produced from TaCl5 and

AlMe3 or AlEt3 at 325 °C and afforded resistivities of 1400 and 700 μΩ·cm, respectively.17

These low resistivity processes for ALD TaCx are in stark contrast to TaN ALD, which generally

produces high resistivity Ta3N5 films because the reduction of precursors in the Ta(V) oxidation

state to the Ta(III) oxidation state in TaN is very difficult.1 _{Other reports of metal carbide ALD}

using AlMe3 have included Nb, Hf, and Ti carbides, but significant incorporation of Al in the

films was also observed.65,185,186

The most commonly used tungsten vapor deposition precursor is WF6, which is a highly

reactive gas that hydrolyzes to form toxic HF.38,40,187–190 WF6 is also reactive toward other

common semiconductor device materials such as Si, Al, or Ti and can lead to spontaneous etching.1,188 Tungsten metal films can be deposited by ALD using WF6 and Si2H6.7,36,38,40,187 The

only reported thermal ALD process for WCx films used WF6 and H2SiEt2.17,24 Although

saturation behavior was not reported, WCx films were deposited with a growth rate of 0.8-0.9

Å/cycle within an ALD window of 150-300 °C and had resistivities between 400-500 μΩ·cm.17,24 _WN

xCy films were deposited by thermal ALD from WF6, NH3, and BEt3 and were

found to be excellent barriers to Cu diffusion.191

Since WF6 is toxic, HF is corrosive, and fluorine impurities may be harmful to device

performance, fluorine-free tungsten precursors have recently received attention.183,192 Tungsten hexachloride (WCl6) is an obviously fluorine-free, comparatively easy to handle volatile solid.

WCx films.50 Herein, this chapter reports the thermal ALD of WCx films with low Al and Cl

impurities using WCl6 and AlMe3. 4.2 Results and Discussion

The ALD process characteristics for WCl6 and AlMe3 were evaluated on a variety of

substrates including Pt, TiN, TaN, Si(100) with native oxide, and 100 nm thermal SiO2

substrates. Similar growth rates were observed on all substrates according to cross-sectional SEM images. All film growth results reported herein were conducted on the 100 nm thermal SiO2 substrates. As shown in Figure 25, self-limited growth was demonstrated for both

precursors at 325 °C, where the growth rate was constant at ~1.5 Å/cycle with pulse lengths of ≥ 0.1 s for AlMe3 and ≥ 2.0 s for WCl6, as measured after 500 cycles. The purge length following

each precursor pulse was kept at 10 s. Due to the high vapor pressure of AlMe3, pulse lengths of

≥ 0.1 s supplied sufficient precursor to afford self-limiting growth. WCl6 was delivered by solid-

state booster at 125 °C, and films deposited using WCl6 pulses of ≥ 2.0 s were in the self-limited

growth regime, as evidenced by their equivalent growth rates and resistivities. (a)

(b)

Figure 25. Plots of growth rate versus precursor pulse length of (a) WCl6 and (b) AlMe3 after 500

cycles at 325 °C.

Using these saturative precursor exposures, the growth rate after 500 cycles was approximately independent of substrate temperature between 275-350 °C, as determined by a plot of growth rate versus substrate temperature (Figure 26). AlMe3 is reported to decompose

thermally at temperatures above 300 °C,193,194 but this decomposition does not appear to affect the growth rate for this process below 350 °C. The growth rate increases outside of the ALD window, which is likely due to increased rate of AlMe3 decomposition at higher temperatures

Figure 26. Plot of growth rate versus substrate temperature after 500 cycles using WCl6 and

AlMe3.

To investigate the growth behavior after differing numbers of ALD cycles, a plot of film thickness versus number of cycles was generated with a substrate temperature of 325 °C using the previously established pulse sequence (Figure 27a). This plot was linear between 200-1000 cycles with a slope corresponding to a growth rate of 1.77 Å/cycle, which is higher than the measured growth rate after 500 cycles of ~1.5 Å/cycle. An “ideal” ALD process would have x-

and y-intercepts at the origin, but in this case they are 69 and -12.275, respectively. These values indicate an apparent nucleation delay at 325 °C on SiO2 of about 69 cycles. When growth rate is plotted versus number of cycles, it is clear that the growth rate increases with increasing number of cycles and likely plateaus at > 1000 cycles (Figure 27b). Nucleation delays are common features in film growth of metals on dielectric substrates.18 In a review of ALD process characteristics, Puurunen discussed the variation of growth rate versus number of cycles.18 _For

the process herein, this observation is consistent with substrate-inhibited growth, which indicates that the number of surface reactive sites is lower on the substrate than on the growing WCx film.

(a)

(b)

Figure 27. (a) Plot of film thickness versus number of cycles at 325 °C. (b) Plot of growth rate versus number of cycles at 325 °C.

X-ray photoelectron spectroscopy (XPS) was used to determine film composition. Analysis of a film deposited at 300 °C showed approximately 1:1 WC stoichiometry with low levels of O, Al, and Cl impurities after argon ion sputtering to remove adventitious carbon and oxygen (Figure 28a). The XPS data are consistent with a WC stoichiometry, and not W2C. The

concentration of Al was below the detection limit, Cl was 1.3 at%, and O was 2.4 at%. The Al concentration increased at 325 and 375 °C to 20.2 and 28.5 at%, respectively. Since AlMe3 is

reported to decompose thermally above 300 °C,193,194 the Al incorporation is likely due to AlMe3

decomposition and is higher with increasing temperature. AlMe3 decomposition is also likely the

cause of the higher growth rate at 375 °C. The high-resolution W 4f and Al 2p XP spectra are shown in Figure 28b and c, respectively. The observed W 4f7/2 binding energy of 31.6 eV and the C 1s binding energy of 283.2 eV agree well with previously reported values for WC.3,12 Another ionization in the C 1s spectrum at a higher binding energy of 284.2 eV may correspond to C-H species or to amorphous C.52,63,185 A possible ionization in the Al 2p region is difficult to discern from the noise, but the Al content is certainly less than 1 at. % at 300 °C.

Figure 28. (a) XPS survey scan of a WCx film deposited at 300 °C after Ar ion sputtering to

remove adventitious carbon and oxygen. High resolution XPS (b) W 4f and (c) Al 2p core level scans of a WCx film deposited at 300 °C after Ar ion sputtering to remove adventious carbon and

Reports of Hf and Ti carbides deposited using AlMe3 had significant Al and Cl

incorporation. At 270 °C, films deposited from HfCl4 and AlMe3 had Al contents between 7.7-

12.5% and Cl contents between 10.1-20%.186 Films deposited between 300-400 °C from TiCl4

and AlMe3 had Al contents between 5-8% and Cl contents between 3-10%.65 Al content was not

reported for NbCx films deposited from NbCl5 and AlMe3, but for films deposited using NbF5

and AlMe3, the Al content was between 2-6%.185 At 300 °C, the F content was ~12% (from

NbF5) and Cl content was ~9% (from NbCl5).185 Regardless of the Nb precursor, the films were

carbon-rich. At 350 °C, the Nb content was 26-28% and the C content was 61-63% as measured by both XPS and Rutherford backscattering spectrometry.185

The structure of the as-deposited WCx films was investigated with grazing incidence X-

ray diffraction. No distinct reflections were observed, indicating amorphous material. WCx films

deposited from WF6 and H2SiEt2, the only other reported thermal tungsten carbide ALD process,

were reported to be polycrystalline.17,24 _{Since diffusion is enhanced along grain boundaries, the}

amorphous films prepared herein could be superior barrier layers to Cu diffusion.

Film roughness was assessed by atomic force microscopy (AFM). Films with thicknesses of 39 and 51 nm were deposited at 325 and 375 °C, respectively, were very smooth and found to have rms roughness values of 0.651 and 0.851 nm, respectively, which corresponds to 1.7% of the film thickness in both cases (Figure 29). Crystalline films tend to be rougher, so these low roughness values correspond well with the observed amorphous structure of the films.

(a)

(b)

Figure 29. AFM micrographs of films deposited from WCl6 and AlMe3 at (a) 300 °C and (b) 375

The resistivity of a 72 nm thick film deposited after 500 cycles at 300 °C on SiO2 was

2770 μΩ·cm. Film resistivity decreased with increasing deposition temperature, reaching a minimum value of 1500 μΩ·cm for an 89 nm thick film at 375 °C (Figure 26). Lower film

resistivity is likely a result of more efficient removal of hydrogen and denser films at higher temperatures. Infrared spectroscopy was used to investigate the possible presence of C-H bonds in a film deposited at 325 °C. No absorbance was observed in the typical C-H stretching region of 2800-3100 cm-1. The lack of a C-H absorption in the infrared spectrum suggests that C-H defects are either not present or at low concentrations. Accordingly, the XPS C 1s ionization at 284.2 eV likely corresponds to amorphous carbon.52,185 For comparison, WCx films deposited by

thermal ALD from WF6 and SiEt2H2 had resistivities between 400-500 μΩ·cm.17,24 The

effectiveness of the ethyl group in producing low resistivity carbide films could be due to efficient β-hydride elimination, which removes ethylene and may produce the metal-rich phase

W2C or metallic W. The Si-H bonds in SiEt2H2 may also serve as a reducing agent for WF6,

which may lead to lower resistivity films.

In this ALD process, AlMe3 behaves as both a carbon source and a reducing agent. Film

growth likely begins with ligand exchange between AlMe3 and surface-bound WClx to afford a

tungsten-methyl surface and volatile byproduct AlMe2Cl (Figure 30). The mechanism of carbide

formation likely involves α-hydride elimination. Similar mechanisms have been proposed in

CVD TiCx and WCx processes using tetrakis(neopentyl)titanium and

tris(neopentyl)(neopentylidyne)tungsten, respectively.52,63,64 By extension of Girolami’s proposed mechanism,63,64 α-hydride elimination from tungsten-methyl bonds would produce tungsten-methylidene and tungsten hydride bonds. Reductive elimination of H2, or possibly CH4,

state in WC. The tungsten-methylidenes can then undergo intramolecular hydrogen transfer to produce methane and WC. Methane production via intramolecular hydrogen transfer was also observed in a surface chemistry study of Ta(NMe)5.195

Figure 30. Possible mechanism of WCx formation.

4.3 Conclusions

Tungsten carbide films were deposited by thermal ALD using WCl6 as the metal

precursor and AlMe3 as both the carbon source and the reducing agent. WCl6 is successfully used

as an ALD precursor for the first time, resulting in a high growth rate of 1.5-1.8 Å/cycle within an ALD window from 275-350 °C. Self-limited growth was confirmed at 325 °C. The as- deposited films were amorphous, which could be advantageous if this material were used as a Cu diffusion barrier. The films were conductive and had a minimum resistivity value of 1500 μΩ·cm

at 375 °C. The film composition was found to have a nearly 1:1 W to C ratio with very low levels of Al, Cl, and O impurities when deposited at 300 °C. At temperatures greater than 300 °C, AlMe3 thermal decomposition leads to increased Al content but does not affect the growth

4.4 Experimental Section

Films for this study were deposited in a Picosun R75-BE ALD reactor operating at 6-8 hPa. WCl6 (99.9%, Strem) was heated to 125 °C in a Picosolid booster to produce adequate

vapor pressure. AlMe3 (97%, Sigma-Aldrich) was delivered at ambient temperature by vapor-

draw ampoule. Nitrogen was used as carrier gas (> 99.999%, Airgas). Films were deposited on 1-2 cm2 _{coupons of SiO}

2 (100 nm)/Si that were rinsed with deionized water and isopropanol and

dried under a stream of nitrogen. Film thickness was measured at a minimum of three points on the films by cross-sectional SEM imaging using a JEOL-7600 field emission scanning electron microscope. Surface roughness was measured on a Bruker BioScope Catalyst atomic force microscope in contact mode with a 2 μm x 2 μm scan size. Grazing incidence X-ray diffraction

patterns were collected on a Bruker D8 Advance diffractometer using a 2° incidence angle. Sheet resistivity measurements were made using a Jandel 4-point probe controlled by a Keithley 2400 Sourcemeter and a Keithley 2182A Nanovoltmeter. X-ray photoelectron spectra were collected using a Kratos Axis Ultra XPS system employing a monochromated Al Kα (1486.6eV) X-ray source at a chamber pressure of 10-9 torr. Argon ion sputtering (4 keV) was used to remove surface adventitious carbon and oxygen. XPS data were analyzed using CasaXPS software. Infrared spectroscopy measurements were taken in transmission mode using a Shimadzu IRTracer-100 spectrometer.